host_buffer.h

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// Copyright 2013 The Flutter Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.
 
#ifndef FLUTTER_IMPELLER_CORE_HOST_BUFFER_H_
#define FLUTTER_IMPELLER_CORE_HOST_BUFFER_H_
 
#include <algorithm>
#include <array>
#include <functional>
#include <memory>
#include <type_traits>
 
#include "impeller/core/allocator.h"
#include "impeller/core/buffer_view.h"
 
namespace impeller {
 
/// Approximately the same size as the max frames in flight.
static const constexpr size_t kHostBufferArenaSize = 4u;
 
/// The host buffer class manages one more 1024 Kb blocks of device buffer
/// allocations.
///
/// These are reset per-frame.
class HostBuffer {
 public:
  static std::shared_ptr<HostBuffer> Create(
      const std::shared_ptr<Allocator>& allocator,
      const std::shared_ptr<const IdleWaiter>& idle_waiter,
      size_t minimum_uniform_alignment);
 
  ~HostBuffer();
 
  //----------------------------------------------------------------------------
  /// @brief      Emplace uniform data onto the host buffer. Ensure that backend
  ///             specific uniform alignment requirements are respected.
  ///
  /// @param[in]  uniform     The uniform struct to emplace onto the buffer.
  ///
  /// @tparam     UniformType The type of the uniform struct.
  ///
  /// @return     The buffer view.
  ///
  template <class UniformType,
            class = std::enable_if_t<std::is_standard_layout_v<UniformType>>>
  [[nodiscard]] BufferView EmplaceUniform(const UniformType& uniform) {
    const auto alignment =
        std::max(alignof(UniformType), GetMinimumUniformAlignment());
    return Emplace(reinterpret_cast<const void*>(&uniform),  // buffer
                   sizeof(UniformType),                      // size
                   alignment                                 // alignment
    );
  }
 
  //----------------------------------------------------------------------------
  /// @brief      Emplace storage buffer data onto the host buffer. Ensure that
  ///             backend specific uniform alignment requirements are respected.
  ///
  /// @param[in]  uniform     The storage buffer to emplace onto the buffer.
  ///
  /// @tparam     StorageBufferType The type of the shader storage buffer.
  ///
  /// @return     The buffer view.
  ///
  template <
      class StorageBufferType,
      class = std::enable_if_t<std::is_standard_layout_v<StorageBufferType>>>
  [[nodiscard]] BufferView EmplaceStorageBuffer(
      const StorageBufferType& buffer) {
    const auto alignment =
        std::max(alignof(StorageBufferType), GetMinimumUniformAlignment());
    return Emplace(&buffer,                    // buffer
                   sizeof(StorageBufferType),  // size
                   alignment                   // alignment
    );
  }
 
  //----------------------------------------------------------------------------
  /// @brief      Emplace non-uniform data (like contiguous vertices) onto the
  ///             host buffer.
  ///
  /// @param[in]  buffer        The buffer data.
  /// @param[in]  alignment     Minimum alignment of the data being emplaced.
  ///
  /// @tparam     BufferType    The type of the buffer data.
  ///
  /// @return     The buffer view.
  ///
  template <class BufferType,
            class = std::enable_if_t<std::is_standard_layout_v<BufferType>>>
  [[nodiscard]] BufferView Emplace(const BufferType& buffer,
                                   size_t alignment = 0) {
    return Emplace(reinterpret_cast<const void*>(&buffer),   // buffer
                   sizeof(BufferType),                       // size
                   std::max(alignment, alignof(BufferType))  // alignment
    );
  }
 
  [[nodiscard]] BufferView Emplace(const void* buffer,
                                   size_t length,
                                   size_t align);
 
  using EmplaceProc = std::function<void(uint8_t* buffer)>;
 
  //----------------------------------------------------------------------------
  /// @brief      Emplaces undefined data onto the managed buffer and gives the
  ///             caller a chance to update it using the specified callback. The
  ///             buffer is guaranteed to have enough space for length bytes. It
  ///             is the responsibility of the caller to not exceed the bounds
  ///             of the buffer returned in the EmplaceProc.
  ///
  /// @param[in]  cb            A callback that will be passed a ptr to the
  ///                           underlying host buffer.
  ///
  /// @return     The buffer view.
  ///
  BufferView Emplace(size_t length, size_t align, const EmplaceProc& cb);
 
  /// Retrieve the minimum uniform buffer alignment in bytes.
  size_t GetMinimumUniformAlignment() const;
 
  //----------------------------------------------------------------------------
  /// @brief Resets the contents of the HostBuffer to nothing so it can be
  ///        reused.
  void Reset();
 
  /// Test only internal state.
  struct TestStateQuery {
    size_t current_frame;
    size_t current_buffer;
    size_t total_buffer_count;
  };
 
  /// @brief Retrieve internal buffer state for test expectations.
  TestStateQuery GetStateForTest();
 
 private:
  [[nodiscard]] std::tuple<Range, std::shared_ptr<DeviceBuffer>, DeviceBuffer*>
  EmplaceInternal(const void* buffer, size_t length);
 
  std::tuple<Range, std::shared_ptr<DeviceBuffer>, DeviceBuffer*>
  EmplaceInternal(size_t length, size_t align, const EmplaceProc& cb);
 
  std::tuple<Range, std::shared_ptr<DeviceBuffer>, DeviceBuffer*>
  EmplaceInternal(const void* buffer, size_t length, size_t align);
 
  size_t GetLength() const { return offset_; }
 
  /// Attempt to create a new internal buffer if the existing capacity is not
  /// sufficient.
  ///
  /// A false return value indicates an unrecoverable allocation failure.
  [[nodiscard]] bool MaybeCreateNewBuffer();
 
  const std::shared_ptr<DeviceBuffer>& GetCurrentBuffer() const;
 
  [[nodiscard]] BufferView Emplace(const void* buffer, size_t length);
 
  explicit HostBuffer(const std::shared_ptr<Allocator>& allocator,
                      const std::shared_ptr<const IdleWaiter>& idle_waiter,
                      size_t minimum_uniform_alignment);
 
  HostBuffer(const HostBuffer&) = delete;
 
  HostBuffer& operator=(const HostBuffer&) = delete;
 
  std::shared_ptr<Allocator> allocator_;
  std::shared_ptr<const IdleWaiter> idle_waiter_;
  std::array<std::vector<std::shared_ptr<DeviceBuffer>>, kHostBufferArenaSize>
      device_buffers_;
  size_t current_buffer_ = 0u;
  size_t offset_ = 0u;
  size_t frame_index_ = 0u;
  size_t minimum_uniform_alignment_ = 0u;
};
 
}  // namespace impeller
 
#endif  // FLUTTER_IMPELLER_CORE_HOST_BUFFER_H_